[docs] [issue38843] Document argparse behaviour when custom namespace object already has the field set

Nov. 19, 2019

      New submission from Ivan Kurnosov <zerkms@zerkms.com>:

At this moment it's impossible to explain the behaviour of this script using documentation.

Given it was explicitly coded to behave like that - it should be somehow noted in the documentation, that as long as a `CliArgs.foo` field has a default value set already - it won't be overwritten with a default argparse argument value.

```
import argparse

class CliArgs(object):
    foo: str = 'not touched'

parser = argparse.ArgumentParser()
parser.add_argument('--foo', default='bar')

args = CliArgs()
parser.parse_args(namespace=args)
print(args.foo) # 'not touched'

print(parser.parse_args()) # 'bar'
```

----------
assignee: docs@python
components: Documentation
messages: 356939
nosy: docs@python, zerkms
priority: normal
severity: normal
status: open
title: Document argparse behaviour when custom namespace object already has the field set

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Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue38843>
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